1. Field of the Invention
This invention relates to oxidation catalysts which are useful in treating exhaust gases from various combustors by decomposing hydrocarbons and carbon monoxide contained in the exhaust gases for complete oxidation into carbon dioxide and water vapor and are also useful in treating relatively light liquid hydrocarbons, such as kerosene, gas oil and the like, and hydrocarbons, such as methane, propane and the like, by catalytic combustion with air. The invention also relates to a method for preparing the oxidation catalysts.
2. Description of the Prior Art
It is known that the platinum group metals, such as platinum, palladium, rhodium and the like, have the highest activity when used as catalysts for completely oxidizing unburnt hydrocarbons in the presence of air into carbon dioxide and water vapor. In application as an oxidation catalyst, the platinum group metals are used as follows. Various inorganic refractory materials, such as cordierite, mullite and the like, are molded in the form of, for example, a honeycomb and inorganic refractory materials having a high surface area, e.g. gamma-alumina, are undercoated on the molding, after which the platinum group metal is deposited on the molding. However, the platinum group metal catalysts are disadvantageous in that they are expensive and it is difficult to stably supply the metals from the standpoint of resources. In addition, the platinum group metal catalysts are not so resistant to heat and thus, when employed at high temperatures not lower than 600.degree. C. over a long term, the catalysts are apt to thermally deteriorate, known as a sintering phenomenon. Accordingly, there is a high demand of development of catalysts which are more resistant to heat.
Oxides of base metals such as cobalt, nickel, iron and the like serve as oxidation catalysts, but are lower in oxidation activity than the platinum group metal catalysts and have a low heat resistance. Thus, these base metal oxides have not been in use as complete oxidation catalysts. In order to overcome these disadvantages, a variety of compound oxides have been recently studied. Especially, extensive studies have been made on compound oxides having a perovskite structure because of the high activity for oxidation. These perovskite compound oxides are represented by the formula, ABO.sub.3, in which A represents a rare earth metal such as lanthanum, neodium or the like, and B represents a transition metal such as cobalt, iron, nickel or the like. In addition, the compound oxides of the formula, AA'BO.sub.3, in which part of A is replaced by A' wherein A' represents cesium, strontium or the like, are known as to improve the activity. It has been reported that when cobalt is used as the transition metal represented by B, the highest activity results.
As is known in the art, rare earth elements are collected from ores which usually contain a mixture of various rare earth elements. However, it is very difficult to separate these rare earth elements from one another, thus inviting high production costs. The proportion in cost of the rare earth elements in perovskite-type catalysts is very high.